1. Field of the Invention
The present invention relates to an optical communication equipment for performing an optical communication with another equipment via an optical communication medium.
2. Description of the Prior Art
In a conventional wavelength-division-multiplexing communication system, several to several tens of light waves are sent by slightly changing their wavelengths, thus allowing a large-capacity communication. It is important to stabilize the oscillation wavelength such large capacity communication.
For this purpose, some methods of stabilizing an oscillation wavelength have been proposed.
For example, in one method, oscillation frequencies are individually assigned to transmission light sources, and respective terminal stations manage the corresponding oscillation frequencies.
In another method, a central station for managing frequencies is arranged, and sends light serving as a predetermined reference to all the subscribers, and respective subscribers tune wavelengths based on the reference light.
As the method of managing frequencies by a central station, a method disclosed in Japanese Patent Laid-Open No. 63-52528 or the like is known.
FIG. 15 is a block diagram showing the principle of the conventional method of managing frequencies by a central station, and FIG. 16 shows an arrangement of a system.
In FIG. 15, reference numeral 7010 denotes a frequency reference light extraction unit for separating frequency reference light from light sent from the central station; 7011, a heterodyne detection unit for performing heterodyne detection of the frequency reference light; 7012, a local oscillation or transmission light source; 7013, a tuner for setting an oscillation frequency of the light source 7012; 7014, a peak detection unit for detecting a peak value of a beat signal obtained by the heterodyne detection unit 7011; and 7015, a frequency position detection unit for detecting a frequency position.
Since a light signal obtained by high-speed modulating oscillation light from, e.g., a stabilized light source is sent as the frequency reference light from the central station to each terminal station, each terminal station causes the frequency reference light extraction unit 7010 to extract the frequency reference light from the light sent from the central station.
An oscillation frequency of the light source 7012 is changed by the tuner 7013, and the heterodyne detection unit 7011 mixes an output from the frequency reference light extraction unit 7010 and an output from the light source 7012, thereby performing heterodyne detection.
The peak detection unit 7014 detects a peak value of a beat signal obtained in detection by the heterodyne detection unit 7011. When the frequency of the light source 7012 is changed by the tuner 7013, a plurality of peak values can be obtained. The frequency position detection unit 7015 compares peak values of adjacent beats, thereby detecting an FM side band of a presently tuned frequency.
In FIG. 16, reference numeral 7020 denotes, a star coupler; 7021, a central station; 7022-1 to 7022-4, terminal stations of subscribers; T.sub.x, a transmitter; and R.sub.x, a receiver.
The terminal stations 7022-1 to 7022-4 are connected by an optical fiber via the star coupler 7020. The central station 7021 is similarly connected via the star coupler 7020.
Multi-frequency transmission using coherent light is then performed. In this case, a reference for setting a frequency is managed not by the terminal stations 7022-1 to 7022-4 individually, but by the central station 7021 systematically, and the central station sends the frequency reference to the terminal stations 7022-1 to 7022-4.
FIG. 17 shows an arrangement of the frequency reference.
In the central station 7021, when oscillation light from the stabilized light source is high-speed modulated at, e.g., 10 GHz to 50 GHz, side bands given by {f.sub.0 .+-.nf.sub.m ] appear around a central frequency f.sub.0, as shown in FIG. 17.
Therefore, the terminal stations detect these side bands, and can determine the positions of the side bands with respect to the central frequency, so that a frequency of a light source used in transmission or reception can be set with reference to the determined position.
For example, many frequencies K.sub.0, K.sub.1, . . . , K.sub.i of the terminal stations can be set between the frequencies f.sub.0 and f.sub.0 +f.sub.m. Therefore, when a reception or transmission frequency band is determined, each terminal station detects a frequency f.sub.0 +f.sub.m of the frequency reference light, thereby setting a frequency offset from these reference frequencies by a predetermined frequency.
However, since frequency bands to be used by light transmitters must be strictly managed and set in order to prevent radio interference, the prior art system suffers from the following drawbacks.
(1) When light transmitters individually have reference wavelength light sources, the temperatures of light-emitting elements of the light transmitters must be strictly controlled to maintain the same reference wavelength.
(2) If a wavelength to be used is assigned in advance to each light transmitter, a given light transmitter cannot use a wavelength assigned to another light transmitter, and use efficiency of wavelengths is impaired.